1. Field of the Invention
The present invention relates to a temperature sensor having a trip temperature detection circuit, and to a method for detecting a trip temperature of a temperature sensor.
2. Description of the Related Art
A dynamic random access memory (DRAM) is a volatile memory in which the memory cells thereof must be periodically refreshed in order to maintain the data stored in the memory cells. Disadvantageously, a relatively large amount of power is consumed during each of these DRAM refresh operations.
It is known that data is preserved longer in DRAM memory cells at lower temperatures. As such, the lower the temperature, the less frequently the memory cells need to be refreshed. Therefore, in an effort to reduce power consumption, the frequency of a refresh clock may be reduced at lower temperatures. Since a lower refresh clock frequency results in fewer refresh operations per unit time, less power is consumed. However, this technique requires the additional provision of a temperature sensor, preferably one which exhibits low power consumption.
FIG. 1 illustrates the circuit configuration of a conventional temperature sensor.
Referring to FIG. 1, the temperature sensor 100 includes a differential amplifier DA which is configured as a type of current mirror having branches A and B. The differential amplifier includes transistors MP1, MN1, MP2 and MN2 as shown, with branch A including a resistance R and a diode D2, and with the branch B including a diode D1. Also provided are transistors MP3 and MN3, as shown, and a third branch C of the temperature sensor is defined by resistance R1. As explained below, a comparator OP1 receives and compares a signal OT1 indicative of a sensed temperature and a signal 0Ref indicative of a reference temperature. The output OUT of the comparator OP1 is high or low depending on whether or not the sensed temperature OT1 exceeds the reference temperature ORef.
The junction diodes D2, D1 of branches A, B have the same diode characteristics. Likewise, the p-type MOS transistors MP1, MP2, MP3 are all of the same size, and the n-type MOS transistors MN1, MN2, MN3 are all of the same size. Here, the term “size” denotes the product of a channel length L and a gate width W of each transistor.
In operation, since the voltage drops across MP2 and MN2 become the same as those across MP1 and MN1, respectively, which become the same as those across MP3 and MN3, respectively, it follows that voltage VA of branch A (i.e., the voltage across R and D2) is the same as the voltage VB of branch B (i.e., the voltage across D1), which is the same as the voltage VC of branch C (i.e., the voltage across R1). Thus, it also follows thatVR+VD2=VD1=VR1where VR is the voltage across the resistance R, VD2 is the voltage across the diode D2, VD1 is the voltage across the diode D1, and VR1 is the voltage across the resistance R1.
A current ID and voltage VD of a junction diode may be generally expressed asID=Is(eVD/VT−1)≈Is(eVD/VT)VD=VT·ln(ID/Is)where Is denotes an inverse saturation current, VD denotes the diode voltage, and VT denotes a thermal voltage. The thermal voltage VT equals kT/q, where k is Boltzmann's constant, T is absolute temperature, and q is electron charge.
From the aforementioned equations, the following relationship can be established:Ir=VT·ln(Ir/IO)/R.Since the thermal voltage VT is proportional to temperature, it follows that the current Ir of branch A is proportional to temperature.
As noted previously, a diode voltage may be generally expressed asVD=VT·ln(ID/Is)Generally, the inverse saturation current Is increases with temperature to a much greater extent than the thermal voltage VT, and accordingly, the diode voltage VD is reduced with an increase in temperature. For this reason, the voltage VD2 of diode D2 decreases with an increase in temperature. Therefore, the voltage VC of branch C also decreases with an increase in temperature, which means that the current I1 is reduced with an increase in temperature.
As such, the current Ir of branch A increases with an increase in temperature, and the current I1 of branch C decreases with an increase in temperature. This relationship is illustrated in FIG. 2, where the axis I denotes current and the axis T denotes temperature. The intersection between Ir and I1 is the trip temperature T1 of the temperature sensor, i.e., the temperature at which OT1 and ORef are the same.
The trip temperature of the sensor can be set by design according to the value of the resistance R1. That is, as shown in FIG. 2, a reduction in the resistance R1 increases the current I1 of branch C, which in turn increases the trip temperature T1 at the cross point of I1 and Ir. In contrast, an increase in resistance R1 decreases the current I1 of branch C, which decreases the trip temperature T1 at the cross point of I1 and Ir. Relying on these relationships, a design value of R1 is selected to achieve a desired trip temperature.
However, the operational characteristics of the temperature sensor of FIG. 1 are highly sensitive to variations in the fabrication processes. Particularly, the actual trip temperature may differ from a designed trip temperature by a certain amount (referred to herein as a “temperature shift”). In order to compensate for this temperature shift, it is necessary to tune the sensor by increasing or decreasing the value of the resistor R1 of branch C. Typically, lasers are used in accordance with well known methods to trim the resistor R1 to the proper resistance level. Preferably, this tuning process is performed at a wafer level for each individual chip.
In order to know the amount of trimming of the resistor R1 that is needed, it is first necessary to know the amount of temperature shift that must be compensated. The conventional technique for determining temperature shift is to place the wafer in a process chamber and vary an interior temperature of the process chamber while monitoring the output signal OUT of the comparator OP1. The chamber temperature at which the output signal OUT changes state is the actual trip temperature of the sensor, and the delta between the actual trip temperature and the design trip temperature is the temperature shift that must be compensated.
Varying the chamber temperature in an attempt to locate the actual trip temperature of the sensor takes a substantial amount of time. Also, the reliability of the temperature measurement and transistor trimming are not always sufficient, and accordingly, it is often necessary to repeat the process of varying chamber temperature to identify the trip temperature after each trimming operation of the resistor R1. In short, the conventional process of setting the trip temperature is very time consuming, thus adversely impacting throughput and costs.